When catalytic converter systems of motor vehicles with internal combustion engines, which are the main focus of this discussion of the prior art, are to be optimized and controlled with regard to the removal of hazardous materials, the first objective is to keep the chronological mean value of the air-fuel ratio lambda, which should clearly fluctuate between values of &gt;1 and &lt;1 in order to ensure proper operation of the catalytic converter system, at a value close to 1, preferably at 1.00.+-.0.02, by controlling the fuel or air supply to the engine, and the second objective is to monitor the reliable operation of the catalytic converter under such controlled conditions.
How one can determine lambda from the signal of an oxygen sensor in any exhaust gas, brought to total chemical equilibrium by knowing the mean chemical composition of the fuel, and thus, to immediately set lambda to an optimal value during engine service or to control it on board, has recently been described (German patent application DE 43 23 879.3). However, measurement and optimization of lambda must be supplemented by monitoring proper operation of the catalytic converter. Monitoring the proper operation can be performed, for instance, with conventional CO, CO.sub.2, NO and HC test instruments based on the infrared absorption method. However, the measurements are rather complex in nature and, therefore, costly and not very suitable for implementation on board motor vehicles.
Observing the aging patterns of catalytic converters of exhaust cleaning systems has revealed that, according to expectations based on physics and chemistry, the effectiveness will first diminish with regard to the conversion of hydrocarbons. "Aged" catalytic converters will continue to largely remove CO and NO, while slow-reacting hydrocarbon molecules will already flow through the catalytic converter to a larger degree without adjustment of the chemical equilibrium.
These realizations lead to the requirement to monitor the functionality of the catalytic converter preferably through the presence of hydrocarbons in the exhaust gas. However, the problem is that not just one well-known hydrocarbon compound will remain in the exhaust gas, but rather mixtures of numerous different hydrocarbons, depending on fuel, temperature in the engine and exhaust pipes (that is, dependent on the engine load), and the activity of the catalytic converter. (The chemical formula CH.sub.x shall be used here to indicate such mixtures.) It is not optimal to measure the concentration of a single compound, but rather to obtain a signal representing the sum of hydrocarbons in the exhaust gas. An additional problem is that the chemical quality of the exhaust gas changes abruptly at lambda=1: CH.sub.x must be detected at lambda&lt;1 in a mixture made up mainly of nitrogen and water gas (CO+CO.sub.2 +H.sub.2 +H.sub.2 O), and at lambda&gt;1 in a mixture made up primarily of nitrogen, water vapor, carbon dioxide and oxygen. The CH.sub.x signal should be independent of the hot exhaust gas, be it reducing (rich gas mixture, lambda&lt;1), neutral (lambda=1) or oxidizing (lean gas mixture, lambda&gt;1). Only with diesel engines can Lambda be expected to be always &gt;1.
Lambda measurements are paramount for car service stations to adjust the engine control and to monitor the exhaust gas for hydrocarbons in order to evaluate the operation of the catalytic converter. Mobile units obtaining gas samples from the exhaust pipes would fulfill the requirement. However, the measurements must be performed at different loads, which would require an expensive engine test stand, or that the measurements be performed during a test drive. For the latter, the instrument must be battery-operated and shock-resistant for on-board use. There is a tendency today, to measure a number of vehicle data continuously through on-board diagnosis and to display these data to the driver. Because of the environmental situation, it is desirable to continuously display lambda and a parameter representing the operation of the catalytic converter, to allow the driver to recognize operating conditions that violate permitted emission values. This would enable the driver to adjust his driving to an environmentally sound style, to follow the aging process of the catalytic converter, and to recognize when the converter needs to be regenerated or replaced.
With stable on-board sensors available to continuously measure lambda and CH.sub.x, the signals of these sensors could also be employed for automatic control or control correction of the engine.
The technical task to achieve the goals outlined above is to supplement the measurement method for lambda in exhaust gases brought into total chemical equilibrium as disclosed in German patent application DE 43 23 879.3 by a method and device such that a signal representing the total concentration of CH.sub.x combinations in the exhaust gas is obtained at any lambda value. It would be desirable to have a device that can be carried on board motor vehicles continuously and that can be used for automatic control, or to correct the control, of engines.